This invention relates to devices having a resonant cavity, and more particularly to a servo system for stabilizing the cavity resonance.
Any finite Q cavity relied upon to have a constant resonance requires a servo-tuning system to compensate for cavity pulling effect, i.e., to compensate for the undesired change from a specified frequency, due to any change in the dimensions of the cavity, particularly a high Q cavity and other effects contributing to long term frequency drift. While reference is made to masers, an acronym for microwave amplification by stimulated emission of radiation, it is in the generic sense that it is used here to refer to any finite Q cavity, even though the specific example described is a maser in the more limited sense.
The acronym laser for light amplification by stimulated emission of radiation has been commonly used to refer to masers that operate at frequencies in the optical region of the electromagnetic spectrum, rather than in the longer wavelength (microwave) region. However, it will be appreciated from the following description of the invention that in theory the present invention is not restricted to the microwave region, although in practice it may be difficult to implement the invention with a shorter wavelength. Consequently, as noted above, the term maser refers to any resonant cavity, including resonant cavities used for such applications or in such devices as klystrons and tuned microwave filters. The practical limits of radiation wavelength will be appreciated, and as technology improves for sensing the phase relationship between the electric and magnetic fields of the radiation inside the cavity, the limit may be extended well into the shorter wavelength of visible light.
The problem with masers has been cavity tuning. Prior-art schemes for servo tuning can adversely affect the performance of the maser. They require either signal injection into the cavity, modulation of the cavity frequency, or modulation of the atomic line Q in order to provide an effect that can be sensed to determine when the cavity frequency has shifted in order to tune the cavity. This injection of a "tracer" of some form capable of being sensed can only have an adverse effect. It would be preferable to use a passive system to sense drift in the cavity frequency, i.e., to sense drift without injection of any tracer, in order to adjust the cavity tuning through a servo, thereby obviating any risk of an adverse effect on the performance of the maser.